The heavily redacted filing also said Flynn, who pleaded guilty a year ago to lying to the FBI about his contact with Russian officials, has helped investigators in relation to three separate enquiries.

While there has been speculation Mr Mueller, under pressure from the White House, may be wrapping up his investigation, Elie Honig, a former federal prosecutor said the document suggested it may continue – at least for some time.

While Mr Honig, a former assistant US attorney for the southern district of New York, has not seen the unrelated version, he said what was visible provided plenty of clues as to how cooperative Flynn had been, and what Mr Mueller had been able to obtain.

“We do not yet know everything about Flynn’s cooperation and the heavy redactions in the filing hide nearly all the details that Mueller provided to the court,” said Mr Honig, now an academic at Rutgers University in New Jersey.

“But, if you read around all that black ink, Mueller did say enough to let us know this: Flynn gave up the goods, and Mueller’s work is far from over.”

In the sentencing document, Mr Mueller said the former army general had provided “substantial” cooperation to investigators “concerning links or coordination between the Russian government and individuals associated with the Trump campaign”.

“His early cooperation was particularly valuable because he was one of the few people with long-term and firsthand insight,” prosecutors wrote. “The defendant deserves credit for accepting responsibility in a timely fashion and substantially assisting the government.”

The legal filing adds to the pressure on Mr Trump, who has repeatedly denied his campaign colluded with Russia and has dismissed Mr Mueller’s investigation as a “witch hunt”.

Later this week, prosecutors are due to provide new information about Mr Trump’s former campaign manager Paul Manafort. Last Friday, the judge in Manafort’s Washington case held a hearing to set a tentative sentencing date after prosecutors accused him of lying during the interviews that occurred after he agreed to plead guilty to financial crimes and cooperate.

The president has already attacked his former legal fixer, Michael Cohen, saying he is making up “stories” to get a reduced prison sentence after his latest guilty plea to lying to congress. The president also praised longtime confidante Roger Stone for saying he would “never testify against Trump,” adding in his tweet: “Nice to know some people still have ‘guts!’”

Last week, Cohen told a court in New York he had lied out of a sense of obligation to the president. “I made these statements to be consistent with Individual-1’s political messaging and out of loyalty to Individual-1,” Cohen said. Individual-1 was identified in court filings as Mr Trump.

Earlier this year, Cohen pleaded guilty in a separate case to to eight charges, including campaign finance violation.

The filing by Mr Mueller, which provides the first real insight into Flynn’s cooperation with investigators probing Russia’s alleged interference, comes two weeks ahead of his scheduled sentencing.

Last December, Flynn, who previously served as director of the Defence Intelligence Agency under Barack Obama, pleaded guilty to one count of lying to the FBI about his contacts with Russian officials, among them Russia’s former US ambassador Sergey Kislyak.

In addition to lying to federal investigators about discussing sanctions with Mr Kislyak, Mr Mueller’s filing says Flynn also lied about interactions with Russians over a United Nations Security Council resolution involving Israeli settlements.

“Each document will tell us something important about what the future holds for these defendants and, more importantly, about what Mueller knows and where he might be headed,” Mr Honig said of the documents that have been filed, or are due to be.

“By Friday’s end, we will know much more about the strength of Mueller’s hand and the threat his investigation poses to President Donald Trump and his administration.”